As a known cap structure (sealing device) used for a container with a upper closure, there is a structure which is provided with an inner plug and an upper closure, and in which a cylindrical part fitted with an opening part of a bottle and a separation part continuously provided on an inner side thereof via a score are provided in the inner plug (refer to FIGS. 1 and 2 in Patent Literature 1).
In this cap structure, the upper closure is preliminarily engaged (screwed) with the separation part of the inner plug through, for example, reverse threads. As a result, a movement difference can be generated between the separation part of the inner plug and the upper closure by screwing backward so as to remove the upper closure from the opening part (forward thread) of the bottle when opening the plug, whereby the separation part is separated from the cylindrical part of the inner plug and the separation part is held by the upper closure. According to this cap structure, the upper closure can be easily opened only by rotating the upper closure so as to be loosened, and the separation part separated by opening of the plug is held on an inner cylinder on an upper end of the upper closure, and thus the separation part after separation can be made to function as a seal, whereby the separation part does not become so-called rubbish.
Incidentally, in assembling of the cap structure in Patent Literature 1, the inner plug is considered to be previously screwed into the inner cylinder of the upper closure, for example, and then, the upper closure is considered to be assembled to the opening part of the bottle together with the inner plug. Here, from a viewpoint of preventing screwing-back or loose rotation of the inner plug, there can be a method of achieving engagement (forced screw-engaging by non-screwing) with each other so that a thread on an inner wall of an outer periphery of the upper closure rides over the thread of the opening part of the bottle, for example. In this case, a height of the thread on the inner wall of the outer periphery of the upper closure and a height of the thread on the opening part of the bottle cannot be made so high. Namely, when these threads are formed high, a large force is required for engaging the upper closure and the opening part during assembling, which makes assembling difficult. On the other hand, when the height of the thread is made low in order to facilitate assembling, an engaging force between the upper closure and the opening part of the bottle becomes weak, and it becomes difficult to screw back the upper closure against resistance when opening the plug. Furthermore, when a relatively large clearance is formed between the thread on the inner wall of the outer periphery of the upper closure and the thread of the separation part, there may be a risk that the upper closure can be easily removed.
On the other hand, as another assembling method, the inner plug is previously fitted in a mouth part of the bottle, and then the upper closure is screwed with the mouth part of the bottle, and the inner cylinder of the upper closure is engaged with the separation part of the inner plug, with the result that the cap can be assembled to the mouth part of the bottle. In this case, when the thread is lowered so that the thread of the inner cylinder of the upper closure reliably rides over the thread of the separation part, there may be cases where the engaging force between the inner cylinder of the upper closure and the separation part becomes weak and the separation part cannot be separated when opening the plug. Moreover, since a relatively large clearance is formed between the thread on the inner cylinder of the upper closure and the thread of the separation part, there is also a risk that the separation part is loosened and is easily removed from the upper closure.